There are nine MAC services specified by IEEE 802.11. These include distribution, integration, asso- ciation, reassociation, disassociation, authentication, deauthentication, privacy, and MSDU delivery.
Six of the services are used to support MSDU delivery between STAs (used as a generic expres- sion for 802.11 devices, both access points and clients). Three of the services are used to control 802.11 WLAN access and confidentiality. Each of the services is supported by one or more MAC frame types. The IEEE 802.11 MAC uses three types of messages: data, management, andcon- trol. Some of the services are supported by MAC management messages and some by MAC data messages. All messages gain access to the WM (Wireless Medium) via the IEEE 802.11 MAC medium access method which includes both contention-based and contention-free channel access methods:Distributed Coordination Function(DCF) andPoint Coordination Function(PCF). In the following, the 802.11 MAC functions and services will be described.
29.4.1 General Medium Access Control Structure
The 802.11 MAC uses a temporal superframe structure with Contention Period (CP)4 and Con- tention Free Period (CFP) alternately as shown in Figure 29.12. Superframes are separated by
4Note that it is only in this section that we use the abbreviation CP for contention period (and not for cyclic prefix). Since we are talking about the MAC layer only, no confusion can arise.
B e a c o n
B e a c o n
CFP (PCF) CP (DCF)
Superframe
Figure 29.12 Superframe structure of 802.11.
Reproduced with permission from IEEE 802.11©IEEE.
periodic management frames, the so-called “beacon frames.” During the CP, DCF is used for channel access, while PCF is used for channel access during the CFP.
The 802.11 MAC uses different interframe gaps, denoted asInter Frame Spaces (IFSs), in order to control medium access – i.e., to give STAs in specific cases a higher or lower priority. These IFSs are (in the order shortest to longest):
• Short Inter Frame Space(SIFS);
• Priority Inter Frame Space(PIFS);
• Distributed Inter Frame Space(DIFS);
• Extended Inter Frame Space (EIFS).
Their actual values depend on PHY parameters.
29.4.2 Frame Formats
The MAC frame format comprises a set of fields that contain various types of control information as well as the actual frame body, all of which occur in a fixed order in all frames. Figure 29.13 depicts the general MAC frame format. The fields Address 2, Address 3, Sequence Control, Address 4, and Frame Body are only present in certain frame types.
Octets: 2 2 6 6 6 2 6 0–2312 4
Frame control
Duration/
ID
Address 1
Address 2
Address 3
Address 4
Frame
body FCS
Sequence control MAC header
Figure 29.13 Medium-access-control frame format (a typical MPDU).
Reproduced with permission from O’Hara and Petrick [2005]©IEEE.
When the MSDUs handed down to the MAC become too large, it becomes difficult to transmit them in one block: obviously, the probability of a block error – i.e., that one of the bits in the block is in error – increases with duration of the block.5 As each block error might lead to the necessity of retransmission, this is highly undesirable. Thus, MSDUs have to be fragmented in order to increase transmission reliability. This fragmentation is done when an MSDU size exceeds the fragmentation threshold. In this case, the MSDU will be broken into multiple fragments with an MPDU size equal to a fragmentation threshold, and a special field (the “more_fragments” field) is set to 1 in all but the last fragment. The receiving STA acknowledges each fragment individually.
The channel is not released until the complete MSDU has been transmitted successfully or until
5Note that, in general, this effect may be offset by the fact that larger blocks allow the use of better codes, like highly efficient LDPC codes. However, for convolutional codes this is not relevant.
Wireless Local Area Networks 747
a nonacknowledgment has been received for a fragment. In the latter case, the source STA will recontend for the channel following the normal rules and retransmit the nonacknowledged fragment, as well as all the subsequent ones.
29.4.3 Packet Radio Multiple Access
Carrier Sense Multiple Access
The DCF employs Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), as described in Chapter 17, plus a random backoff mechanism. Support for DCF is mandatory for all STAs. In DCF mode, each STA checks whether the channel is idle before attempting to transmit.
If the channel has been sensed idle for a DIFS period, transmission can begin immediately. If the channel is determined to be busy, the STA will defer until the end of the current transmission.
After the end of the current transmission, the STA will select a random number called a “backoff timer,” in the range between 0 and aContention Window(CW). This is the time the WM has to be free before the STA might try to transmit again. The size of the CW increases (up to a limit) every time a transmission has to be deferred. If transmission is not successful, the STA thinks that a collision has occurred. Also in this case, the CW is doubled, and a new backoff procedure starts again. The process will continue until transmission is not successful (or discarded). The basic access method and backoff procedure are shown in Figures 29.14 and 29.15, respectively.
Immediate access when medium is free >= DIFS DIFS
DIFS
SIFS PIFS
Contention window
Backoff-window Slot time
Select slot and decrement backoff as long as medium is idle
Next frame
Defer access Busy medium
Figure 29.14 Basic access method.
Reproduced with permission from IEEE 802.11©IEEE.
Physical and virtual carrier-sense functions are used to determine the state of the channel. When either function indicates a busy channel, the channel should be considered busy; otherwise, it should be considered idle. A physical carrier-sense mechanism is provided by the PHY. A virtual carrier- sense mechanism is provided by the MAC. This mechanism is referred to as theNetwork Allocation Vector (NAV). The NAV maintains a prediction of future traffic on the medium based on duration information that is announced in the DURATION field in the transmitted frames.
Polling
PCF is an optional medium access mode for 802.11. It provides contention-free frame transfer, based on polling (see Chapter 17). ThePoint Coordinator (PC) resides in the BS (access point).
All STAs inherently obey the medium access rules of the PCF and set their NAV at the beginning of each CFP. The PCF relies on the PC to perform polling, and enables polled STAs to transmit without contending for the channel. When polled by the PC, an STA transmits only one MPDU, which can be to any destination. If the transmitted dataframe is not in turn acknowledged, the STA
Station A
DIFS Frame CWindow
Backoff Defer
Defer
Frame Defer
Defer
CWindow
CWindow
CWindow
Backoff
Remaining backoff CWindow = Contention window Frame
Frame
Frame Station B
Station C Station D Station E
Figure 29.15 Timing backoff procedure.
Reproduced with permission from IEEE 802.11©IEEE.
does not retransmit the frame unless it is polled again by the PC, or it decides to retransmit during the CP. An example of PCF frame transfer is given in Figure 29.16. At the beginning of each CFP, the PC senses and makes sure the channel is idle for one PIFS before sending the beacon frame.
All STAs adjust their NAVs according to the broadcast CFP duration value in the beacon. After one SIFS time of the beacon, the PC may send out aContention-free Poll (CF-Poll), data, or data plus a CF-Poll. Each polled STA can get a chance to transmit to another STA or respond to the PC after one SIFS with an acknowledgment (plus possibly data).
Contention-free repetition interval Contention-free period
SIFS SIFS SIFS PIFS SIFS
Beacon D1 + poll
U1 + ACK U2
+ ACK U4 + ACK
D3 + ACK +po l
D4 + poll
SIFS PIFS
NAV
SIFS SIFS
CF-End
CF_Max_Duration Reset NAV
Contention period Dx = Frames sent by point coordinator Ux = Frames sent by polled stations No
response to CF–poll D2 + ACK
+poll
Figure 29.16 Point-coordination-function frame transfer.
Reproduced with permission from IEEE 802.11©IEEE.
As discussed above, the DCF and PCF coexist in a manner that permits both to operate concur- rently.6 The two access methods alternate, with a CFP followed by a CP. Since the PCF is built on top of the DCF, there are no conflicts between DCF and PCF when they coexist in the system.
All STAs will inherently obey the medium access rules of the PCF. STAs will stay silent during CFP unless they are polled.
6Within the same Basic Service Set (BSS).
Wireless Local Area Networks 749